1. Field of the Invention
This invention relates to multicast groups in a computer network, and more particularly to transmission of multicast groups from a router through a layer 2 switch to another router, which then distributes the packets to further downstream routers or to end stations.
2. Background Information
A network exchange point is often designed with a Layer 2 switch interconnecting a plurality of fast routers. The Layer 2 switch has a plurality of ports, and usually one router is connected to one port of the Layer 2 switch. Also, an end station may be directly connected to the Layer 2 switch.
An end node is spoken of as “desiring” to receive multicast packets of a particular multicast group, for example, in response to a user (person) executing a command to begin receiving the multicast group packets. An end node desiring to receive multicast group packets sends a relatively simple packet, referred to as an Internet Group Management Packet (IGMP) packet, to the first upstream router in order to execute a relatively simple multicast group join procedure. A router, in response to receiving an IGMP join packet, sends a Protocol Independent Multicast (PIM) join packet to other routers in order to execute a relatively complex multicast join procedure referred to as a “PIM join”. The PIM join packet informs other routers that the new router requests multicast group traffic from a particular group. The IGMP protocol is described in RFC 1112 and RFC 2236, and the PIM protocol is described in RFC 2117 and RFC 2362, where an RFC is a Request for Comments published by the Internet Engineering Task Force, and all disclosures of these RFCs are incorporated herein by reference. A copy of an RFC may be downloaded from the IETF Web Site located at URL www.ietf.org.
In the event that a layer 2 switch interconnects the routers, a PIM join packet transmitted by a router and reaching the layer 2 switch is forwarded to each of the ports of the layer 2 switch connected to a multicast router. The PIM join packet is then ordinarily transmitted by the port to the multicast router. Routers receiving the PIM join packet respond in accordance with the PIM protocol to set up the multicast group distribution tree. A layer 2 switch forwards all multicast packets to all ports connected to a router, except it does not forward the multicast packet through the port from which the multicast packet arrived.
Multicast data traffic may arrive at the Layer 2 switch from any one of the multicast routers connected to it. The Layer 2 switch then transmits all multicast data packets through each port connected to a multicast router. That is, the Layer 2 switch does not discriminate between different multicast groups and the routers which need to receive packets of a particular group. The Layer 2 switch simply transmits all multicast group packets to all multicast routers by transmitting multicast packets through the ports connected to the multicast routers.
Accordingly, each of the routers receives multicast group packets which it does not need. This redundant traffic is unwanted.
In the event that an end station is connected to a layer 2 switch, the IGMP packet which the end station sends to a router passes through the layer 2 switch. The layer 2 switch, if enabled, may use “IGMP snooping” to examine the contents of an IGMP packet. In examining the contents of an IGMP packet, the layer 2 switch reads the group address, and in response, forwards only multicast traffic of the group requested to the end station.
There is needed a method to have a Layer 2 switch transmit multicast group traffic to only those routers which need packets of a particular multicast group.